1. Field of the Invention
This invention relates to a material for packaging photosensitive materials, and more particularly to a less expensive packaging material which combines, in an optimum manner, the characteristics required of a packaging material, i.e., physical strength, light-screening properties, moisture barrier characteristics, antistatic properties and heat sealability.
2. Description of the Prior Art
A photosensitive material and other materials which lose their commercial value upon exposure to light are usually packaged in a bag which completely shields them to light. The bag is required to have satisfactory breaking strength, tear strength, bursting strength, surface strength, seam strength, and other physical characteristics that vary according to the size and weight of the contents. If the bag is used to pack a photosensitive material, it is also required to have satisfactory heat sealability and heat sealing strength because it must be heat sealed to close completely the opening for light- and moisture-shielding purposes. As a further requirement, the bag must have antistatic properties that eliminate the chance of electrical charges building up due to friction between the photosensitive material and the bag and damaging the photographic material by the formation of static marks on the film.
Heretofore, a packaging bag that virtually satisfies the requirements of light-shielding performance, physical strength, heat sealability and antistatic properties has been prepared using a composite laminate wherein a low-density polyethylene film incorporating carbon black and pigments is combined with paper, aluminum foil, cellophane, etc. The current popularity of such a complex laminate as a packaging material is due to:
(1) Light-shielding performance: The low-density polyethylene film accommodates more carbon black and the like as a light screen, thus requiring only a thin film to achieve the desired light-screening effect.
(2) Moisture barrier characteristics: Because of its high air permeability, a low-density polyethylene film must be rendered significantly thick or laminated with an aluminum foil to have the desired moisture barrier characteristic. However, only a slight increase in thickness may be required of a high-density polyethylene film whose moisture permeability is about a third that of the low-density polyethylene film.
(3) Physical strength: The tear strength of the bag is increased by laminating the low-density polyethylene film with paper, incorporating a synthetic rubber in the film, or by increasing the overall thickness of the film. A packaging material for use with materials other than photosensitive materials has been prepared by knitting two kinds of high-density polyethylene flat yarns stretched in different directions (as disclosed in U.S. Pat. Nos. 3,445,055 and 3,816,230), by passing a thermoplastic film between embossing rolls to provide a mesh heat sealed pattern (as disclosed in U.S. Pat. No. 3,973,063), or by bonding, through the medium of an intermediate layer, two high-density polyethylene films stretched in different directions (as disclosed in U.S. Pat. No. 3,322,613).
(4) Antistatic properties: Addition of carbon black also results in rendering the polyethylene film antistatic. The same purpose can be achieved by laminating the film with aluminum foil.
Studies are also being made on combining two high-density polyethylene films stretched in different directions and providing the combination with light-shielding, moisture barrier and antistatic properties for use as a material for packing a photosensitive material. U.S. Pat. No. 4,147,291 entitled "Packaging Bag for Light-sensitive Material" filed by the applicant of the present application is one of the outcomes of these studies.